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When designing your database, and your document structure, there are a number of
best practices to take into consideration. Especially for people accustomed to
relational databases, some of these techniques may be non-obvious.

While CouchDB will generate a unique identifier for the _id field of any doc
that you create, in most cases you are better off generating them yourself for
a few reasons:

If for any reason you miss the 200OK reply from CouchDB, and storing the
document is attempted again, you would end up with the same document content
stored under duplicate _ids. This could easily happen with intermediary
proxies and cache systems that may not inform developers that the failed
transaction is being retried.

_ids are are the only unique enforced value within CouchDB so you might
as well make use of this. CouchDB stores its documents in a B+ tree. Each
additional or updated document is stored as a leaf node, and may require
re-writing intermediary and parent nodes. You may be able to take advantage of
sequencing your own ids more effectively than the automatically generated ids
if you can arrange them to be sequential yourself.

Because of replication, as well as the distributed nature of CouchDB, it is not
practical to use auto-incrementing sequences with CouchDB. These are often used
to ensure unique identifiers for each row in a database table. CouchDB generates
unique ids on its own and you can specify your own as well, so you don’t really
need a sequence here. If you use a sequence for something else, you will be
better off finding another way to express it in CouchDB in another way.